Since the early days of electrotechnics, printed circuit boards have been in the form of standard singlelayer or multilayer "printed" circuits formed from a combination of flat current parts (connections of the electronic circuit) and an insulating plate (the electrically insulating, mechanical support). The structuring of the circuit (layout) takes place by photochemical processes. Apart from surface-mounted devices or SMD technology, the circuit boards have passages, which are used on the one hand for the through-assembly (electrical contacting and mechanical fixing) of simple electronic components (such as resistors, capacitors, coils, etc.) and on the other hand are used to electrically connect current paths of different layers of a circuit board by means of plated-through holes.
However, the development of more complex circuits has taken place on the plane of integrated circuits (IC's), which comprise a plurality of electronic elements (transistors) on a common substrate and which are protected by a casing (chips). These chips have a very high density and ever-increasing compression with respect to circuits, which largely takes place through the miniaturization or reduction of the conductor thicknesses and the conductor spacings in the Si-substrate in a sub-.mu.m range. The casing dimension of such chips remains of the same order of magnitude as the aforementioned simple components, for easy handling reasons, while the dimensions of the circuit board and their holes or plated-through holes has remained largely unchanged. Thus, the miniaturization of circuits has mainly taken place by means of integrated circuits, which are independent, function-linked units, which can be easily inserted on circuit boards.
The structural dimensioning of the printed circuit boards has only incompletely participated in this miniaturization. There has indeed been a certain miniaturization of circuit boards with conventional technology, but this has occurred less through rigorous compression than by the greatest possible narrowing of the conductors.
However, significant advances have been prevented by the unavoidable plated-through holes, which have hitherto been subject to precision improvements, but not to miniaturization. The reason for this is that the circuit board plated-through holes have reached a structurally caused lower limit and cannot undergo further size reductions because, apart from the drill diameter limit, a considerable amount of space is taken up by the soldering pads and the prepared solder surfaces. However, it would be desirable to manufacture substrates (circuit boards) with a ten times greater density, but this is not possible with the gradual miniaturization of standard technology. Such a dimensioning jump leads to conflicts of aims.
Circuit boards, prepregs and conductor films are generally 0.1 to 1 or 2 mm thick and their hole diameter is necessarily 0.2 to 0.5 mm. Such dimensions are consequently 2 to 3 orders of magnitude (100 to 1000 times) above the corresponding dimensions of the conductor thicknesses and spacings in integrated circuits. There is a very significant dimensional variation between circuit boards and IC's.
The reason for the conventional and to some extent archaic construction of circuit boards is that it was not hitherto necessary to make significant innovations in circuit board technology with respect to a downward dimensioning. The presently conventional construction also has a technical basis. In order to be able to support components, the circuit boards must have a certain strength and it has not hitherto been possible to get away from this. Limits are placed on the mechanical drilling for the production of plated-through holes, e.g. with the presently minimum drill diameters of 0.2 mm, while the size and positional accuracy of galvanically through-plated holes are also limited.
This stagnation of conventional manufacturing technology relative to circuit boards, accompanied by a sweeping development relative to integrated components, has unavoidably led to the aforementioned structural problems. Complex chips lead to evermore complicated arrangements on the circuit board. The dimensions of the current paths, holes and plated-through holes have remained unchanged for some time. As a result up to 60-layer circuit boards are used, whose layout causes high expenditure and effort and whose manufacturing costs constantly rise. The miniaturization of complete circuits is made more difficult by the conflict of aims of the chips, which require denser integration, and the circuit boards, which come up against the layout miniaturization limit and are no longer able to meet demands. A way out by increasing the number of layers for multichip modules (MCM) in circuit boards as a result of the compression of the chips leads to excessive manufacturing cost rises compared with the manufacturing costs of the actual chips, which constitute the core of the overall circuit.
Miniaturization is also no longer restricted to the smallest sizes of the conductor spacings in integrated circuits and also applies to electrical and electronic equipment. Small pocket televisions, portable personal computers (laptop), hand cameras and other highly complex equipment necessarily require small circuit boards and conductor foils. It is often necessary to adapt the shape and design of circuit boards to the equipment and flexible conductor foils are particularly suitable for this.
Roll-to-roll processing of foils is possible, which permits a high degree of automation. In addition, the continuous plants used with conventional materials e.g. for developing photoimages, etching, stripping, brushing, etc. are always conveyed with a conveying system (progressive assembly line, clamps), conveying the rigid boards through the machine. In the case of a foil there is no need for such a conveying system, which not only leads to plant cost savings, but also eliminates the conveying system, which often produces defects on the product, such as scratches, pressure points, etc.
In addition, foils do not generate dust, which is a further important advantage. In the case of rigid materials dust is released from the cut edges and this leads to reduced output. As a result of the planeness and smooth surface, the foil can be very easily cleaned. In the rolled up state the foil material surface is also protected against contamination, particularly dust.
Unlike in the case of conventional material, a foil is produced in a continuous process, i.e. the material has constant parameter values along its web. In particular, the shrinkage and/or elongation behavior is constant and can therefore be easily compensated. Unlike in the case of rigid, glass fiber fabric-reinforced material, the foil surface is very flat or smooth and is not surface-modulated by the glass fiber fabric. Thus, in the photochemical transfer of images, particularly during exposure, better production characteristics are achievable. Moreover, as the foil is flexible, it can adapt closely to the photomask, which is also a film, which avoids so-called hollow exposures, which naturally leads to improved output.
Through the use of a foil it is possible to cover both rigid and flexible uses. In addition, rigid, flexible combinations are possible. When using a thin foil the resulting hole diameter is very large compared with the hole length or depth, when said ratio is compared with conventional circuit boards. A 25 .mu.m thick foil with a relatively small hole of only 80 .mu.m has a ratio of more than 1:3. This is very advantageous in the case of electrogalvanic feed-through, because the material exchange from the hole to the surrounding electrolyte functions much better than in thin, long holes of a conventional nature in circuit boards, which are very difficult to galvanically feed-through.
With respect to the thermal expansion occurring in the Z-direction no problems are caused by a thin foil. Conventional 1 to 5 mm thick printed circuit boards have in the Z-direction a thermal expansion which must not be underestimated and which can lead to tearing of the galvanic coating introduced into the holes, which represents an electrical defect.
In the case of electronic circuits with a high packing density heat dissipation is also usually a very serious problem. In conventional circuit boards so-called heat sinks are laminated in or laminated onto the surface. They consist of thermally good conducting materials such as copper or aluminium and serve to pass the thermal energy produced by the components as efficiently as possible to the cool points such as e.g. the casing wall. As in conventional technology the circuit board is usually 1 to 5 mm thick, this means that the thermal energy generated by the components must firstly be passed through the circuit board to the cooling plate. As the circuit board primarily consists of a poor heat conductor (plastic, glass), this leads to a high thermal resistance between the heat-producing component and the cooling place. Thus, in many cases it is necessary to provide special plated-through holes (thermal vias) for heat conduction purposes and they take up a considerable amount of expensive space. As a result, in many cases, further conductor layers are necessary, which in turn further increases the circuit board thickness.
When using thin foils even under the most extreme packing and connection densities (conductors), the circuit board is only roughly 0.1 to 0.2 mm thick. Therefore this foil is a comparatively small thermal resistance and therefore improves the thermal management to a significant extent.
If for the construction of the electronic circuit use is made of leadless ceramic chip carriers (LCCC's) or larger ceramic capacitors and other components made from a material with a small thermal expansion coefficient and they are soldered directly to the circuit board without any elastic connection to the substrate (e.g. elastic contact legs), then to obtain a sufficient reliability of the solder points the circuit board would have to undergo a thermal expansion behavior adaptation. Conventionally use is made of rolled metal foils made from copper-invar-copper (CIC) or coppermolybdenum-copper (CMC) with an expansion coefficient of approximately 4 to 6 ppm/.degree.K. Uses are also known with carbon fiber composite materials or aramide fiber composite materials.
In conventional technology the non-reinforced circuit boards are in the range 16 to 18 ppm/.degree.K. and, as the plates are usually between 1 and 5 mm thick, they must be stabilized by a large amount of CIC, CMC, etc. This leads to a significant increase in the total circuit board thickness, which negatively influences the reliability and also makes thermal management more difficult. In addition, such circuit boards become relatively heavy, which is certainly undesired in the avionics field.
However, if foils are used as the base material, the total thickness of a complex circuit board is approximately 0.1 to 0.2 mm. If this foil circuit board is laminated onto a support, which has a low thermal expansion coefficient, then the expansion coefficient of the circuit board surface is almost identical to that of the support plate or board, because the thin foil has no significant influence on the overall structure. The foil is also not reinforced by glass fibers, so that the modulus of elasticity of the foil is much lower than that of glass fiber-reinforced materials and is consequently not negatively influenced by the stabilizing action of the support plate.
If within an electronic circuit use is made of high frequency signals, then an important part is played by the so-called impedance (i.e. wave impedance) of the electric lines. Thus, for avoiding reflections, use is made of lines with a 50 .OMEGA. or higher impedance (standard). Apart from the absolute size of the wave impedance, great importance is attached to constancy over the conductor length for high frequency characteristics. An important part is played by the dielectric thickness constancy. The use of a foil offers much better tolerances with respect to the dielectric constant and the material thickness as compared with conventional base materials.